Drilling fluid is used for various purposes when drilling wells. For instance, drilling fluid may be used to cool the drill bit or to flush away debris (e.g., rock cuttings) from the vicinity of the drill bit, thereby promoting drill bit longevity and optimal performance. Various factors may be considered when determining a desired flow rate for drilling fluid near the drill bit in a particular drilling environment—for example, the desired rate of penetration, mud density, and mud viscosity, among others.
Achieving the desired fluid flow rate, however, can be challenging. Fluid circulation systems that transport drilling fluid from the surface pump, to the drill bit, and back to the pump generally have variable pressure gradients, and this variability results in flow rate unpredictability. Contributing to this unpredictability are volume effects due to weight on the drill bit and torque forces that affect long drill strings (e.g., thousands of feet); these volume effects affect the pressure of fluid traveling through the drill string and, by extension, the fluid flow rate. In addition, long drill strings present delays between the time a particular speed or torque setting is applied to the pump and the time that the pump setting affects the fluid flow rate at the drill bit. Thus, controlling the pump speed and torque with the goal of achieving a desired fluid flow rate at the drill bit often produces unintended outcomes.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.